FTP: prevent the multi interface from blocking

As pointed out in Bug report #3579064, curl_multi_perform() would
wrongly use a blocking mechanism internally for some commands which
could lead to for example a very long block if the LIST response never
showed.

The solution was to make sure to properly continue to use the multi
interface non-blocking state machine.

The new test 1501 verifies the fix.

Bug: http://curl.haxx.se/bug/view.cgi?id=3579064
Reported by: Guido Berhoerster

tests/data/test1501

@@ -0,0 +1,53 @@
+<testcase>
+<info>
+<keywords>
+FTP
+RETR
+multi
+LIST
+</keywords>
+</info>
+
+# Server-side
+<reply>
+<data>
+</data>
+<servercmd>
+DELAY LIST 2
+DELAY TYPE 2
+</servercmd>
+</reply>
+
+# Client-side
+<client>
+<server>
+ftp
+</server>
+<tool>
+lib1501
+</tool>
+ <name>
+FTP with multi interface and slow LIST response 
+ </name>
+ <command>
+ftp://%HOSTIP:%FTPPORT/1501/
+</command>
+</client>
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+0
+</errorcode>
+<protocol>
+USER anonymous
+PASS ftp@example.com
+PWD
+CWD 1501
+EPSV
+TYPE A
+LIST
+QUIT
+</protocol>
+
+</verify>
+</testcase>